1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a downhole hybrid cable, and more particularly to a downhole hybrid cable that has both fiber and copper elements.
2. Description of the Related Art
Hybrid cables with fiber and a copper wire are used for various purposes. For example, they are used for supplying power via the copper wire while sensing is carried out on the fiber. Also, sensing can be carried out via the copper wire as well. Such hybrid cables have also been employed in logging cables for downhole use. The logging cables are meant to be put into, for instance, an oil well to collect sample measurements of the well structure. After completion of the measurements, and verifying that the data has been collected, the logging cable is pulled out of the oil well.
The existing technology for downhole hybrid type cables that have both fiber and copper elements includes (1) a center fiber/gel filled stainless steel tube with copper wire wrapped around the tube and an insulation layer around the copper wire/tube configuration which is produced by Gulf Coast Downhole Technologies located in Houston, Tex. Another existing structure (2) has a center insulated copper wire with small plastic fiber/gel filled tubes with an insulation around it. This structure (2) is made by Draka.
The disadvantage of item (1) is that the copper wire 6 is not easily segregated from the stainless steel tube. Attaching sensing elements to the cable when the cable is terminated, i.e. stripped back, is a taxing procedure. The user needs to ensure that the copper wire is separated from the stainless steel tube and it has to be re-insulated as the insulation has to be removed to get to the copper wires. Another disadvantage is that the center stainless steel tube has to be of such a size that the excess fiber length (EFL) in the tube must be relatively low, in the case where a multi-mode optical fiber is deployed in it. This fiber is commonly used for temperature sensing, so it is often used in this type of tube. Single mode optical fiber is also used in well for sensing. It is less sensitive than multi-mode optical fiber so the excess fiber can be slightly higher but given that multi-mode and single mode optical fiber is commonly deployed in the same cable, the excess fiber length will be driven by the multi-mode fiber. If the stainless steel tube is approximately 0.080 inches or smaller, then the EFL can only be 0.10 to 0.15% with respect to the length of the fiber in the core in order to still have good optical performance. This limits the amount of strain that the cable can see before the fiber is also under strain. This can be an issue for environments where the cable temperature will be elevated.
More particularly, in downhole fiber optic cables, a ¼″ metal tube is used to house the fiber optic core. With this diameter and the ¼″ tube's wall thickness, typically 0.028″ or 0.035″, the inside diameter of the ¼″ metal tube is fixed. This results in the cable designer needing to work in a small space to house the desired copper and fiber elements. In order to fit a 0.080 inch fiber filled stainless steel tube into this ¼″ tube and to include copper elements with the appropriate insulation level to ensure proper performance of the copper, the size of the stainless steel tube is limited.
In general, as the stainless steel tube size increases, more excess fiber can be put into it and still have acceptable optical performance (too much excess fiber can create optical loss). Excess fiber is needed in the stainless steel tube to ensure good optical performance during temperature changes in, for example, the oil well. As the temperature increases, the metal expands faster than the fiber, and in the case that there is no excess fiber in the stainless steel tube, the fiber would be under strain as the temperature increased. Increased strain reduces fiber life, can increase attenuation (optical loss), and can affect other attributes on the fiber. In the unitube configuration of item (1), with copper wire wrapped around the tube, the geometry is such that the center stainless steel tube is small, i.e., 0.080 inches or less. This is a drawback to this type of design since the center stainless steel tube size limits the EFL in the tube.
Item (2) overcomes the EFL issues of item number 1 by stranding the plastic tubes around the insulated copper wire. However, due to the size of the plastic tubes, the amount of benefit is limited. The stranding provides for radial movement of the fibers in the tube which increases the amount of cable strain experienced by the plastic tubes before the fiber sees strain. However, with this structure, the disadvantage is that the inherent strength of the structure is limited because the strength element of the structure is only the center copper wire. This becomes problematic as processing tensions on the core and installation practices can result in high tension levels on the cable, thus exposing the fiber to strain. Another disadvantage of item (2) is its crush resistance. The plastic tube is limited in the amount of external force that can be applied to it, in order to still have good optical performance.